Process for producing crosslinked, melt-shaped articles

ABSTRACT

Crosslinked, melt-shaped articles are manufactured by a process that does not require the use of post-shaping external heat or moisture, the process comprising the steps of: A. Forming a crosslinkable mixture of a 1. Organopolysiloxane containing one or more functional end groups; and 2. Silane-grafted or silane-copolymerized polyolefin; and B. Melt-shaping and partially crosslinking the mixture; and C. Cooling and continuing crosslinking the melt-shaped article. Crosslinking is promoted by the addition of a catalyst to the mixture before or during melt-shaping or to the melt-shaped article.

PRIORITY

This application claims priority to U.S. Patent Application No.61/242,857 filed on Sep. 16, 2009, the entire content of which isincorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to crosslinked, melt-shaped articles. In oneaspect, the invention relates to a process for producing crosslinked,melt-shaped articles while in another aspect, the invention relates tosuch a process in which the articles are crosslinked using anorganopolysiloxane containing two or more functional end groups. In yetanother aspect, the invention relates to such a process in which thecrosslinking is accomplished without requiring the use of post-shapingexternal heat or moisture.

BACKGROUND OF THE INVENTION

Compositions used in the manufacture of crosslinkable articles, such asheat resistant wire & cable coatings and molded parts and accessories,typically require cross-linking after final shaping. Variouscrosslinking methods are practiced in the art, two of which are in wideusage, i.e., peroxide crosslinking and moisture cure (the latter ofwhich usually employs a silane grafted or copolymerized polyolefin).

Moisture cure systems have the advantage in that they can be processedwithin a wide range of melt temperatures but are generally limited tothin wall constructions because the crosslinking relies on diffusion ofexternal moisture into the article. Peroxide cure compositions arepreferred for thick wall constructions, e.g. medium voltage (MV) cableinsulation and molded cable accessories. These curable compounds need tobe processed at temperatures which are below the peroxide decompositiontemperature in order to avoid premature crosslinking (scorch) prior toforming the article. Once the article is formed, it needs to be heateduniformly to the peroxide decomposition temperature, and then held atthat temperature for the time necessary to achieve the desired level ofcrosslinking. This can keep the production rate for such articles lowdue to poor heat transfer through the article walls. Furthermore, oncethe article is cooled, peroxide decomposition slows down to negligiblelevels; thus any significant crosslinking comes to an end. The combinedproblems of scorch and long heating and cure times (whether in-mold curetime or residence time in a continuous vulcanization tube) lead to longmanufacturing cycles, and thus low productivity (units per time).

BRIEF SUMMARY OF THE INVENTION

In one embodiment the invention is a process for the manufacture ofcrosslinked, melt-shaped articles, the process comprising the steps of:

A. Forming a crosslinkable mixture comprising:

-   -   1. Organopolysiloxane containing two or more functional end        groups; and    -   2. Silane-grafted or silane-copolymerized polyolefin;

B. Melt-shaping and partially crosslinking the mixture into an article;and

C. Cooling and continuing crosslinking the melt-shaped article.

The process does not require the use of post-shaping external heatand/or moisture although either or both can be used if desired.Crosslinking can be promoted by the addition of a catalyst to themixture before or during melt-shaping, or to the melt-shaped article(e.g., by diffusion from an adjoining layer if the article is a layer ina multilayer construction. Surprisingly, compounding a mixturecontaining these components produces a stable thermoplastic compositionwhich can be shaped and partially crosslinked by melt processing into anarticle, but upon storage at ambient conditions undergoes thoroughcrosslinking without the need for external moisture or heat. At amicroscopic scale the morphology of such a blend shows greatercompatibility between the silicone and the polyolefin phases compared toeither a physical (unreacted) siloxane/polyolefin blend or a physical,i.e., unreacted, blend of a siloxane and a silane-grafted polyolefin.

The process of this invention eliminates the reliance on externalmoisture diffusion that is required in conventional moisture cure. Theprocess of this invention is particularly useful for manufacturingthick-wall (greater than (>) 0.2, more typically >0.5 and even moretypically >1, millimeter (mm)), crosslinked constructions such as inhigh and medium voltage cable insulation, wire and cable moldedelastomeric connectors and accessories, and molded automotive heatresistant parts. In the case of injection molded parts, after injectionin a mold and once the article is formed, the compositions do notrequire additional heating or holding times to cure. Rather, the articlecan be cooled to achieve green strength to retain the desired shape asis common in thermoplastic injection molding operations. Once removedfrom the mold, the cure step continues off mold to achieve full cure.This approach improves manufacturing cycle time and achieves higherproductivity (units per time).

In one embodiment hydroxyl-terminated silicone is reacted with an alkoxysilane (or silanol) that is grafted to a polyolefin or other polymer.Methods for preparation of such grafted polymers are well known. Forexample, vinyltrimethoxysilane (VTMS) can be grafted to polyethyleneusing peroxide. Also, various reactor copolymers are available, such asSI-LINK™, which is a copolymer of VTMS and ethylene available from TheDow Chemical Company.

Silicone polymers with hydroxyl end groups are readily available.Reactions of these silicones directly with grafted alkoxysilanes orsilanols provide an interesting range of approaches, including:

A. Crosslinking via direct reaction (at high levels for networkformation or low level coupling for melt strength enhancement throughlong chain branches);

B. Formation of silicone-functionalized polyolefins by operating underconditions that do not result in formation of a crosslinked network(e.g. use of monohydroxyl silicone or very low levels of dihydroxysilicone, or low graft levels on the polymer); if a suitable amount ofSiOR remains in the system after functionalization, subsequent moisturecrosslinking is possible; and

C. Silane-grafted polyolefins can be dynamically crosslinked in thepresence of polyolefins that do not contain grafted silane to makethermoplastic vulcanizates (TPV) using silicone-mediated crosslinkingreactions.

In one embodiment the invention is a process for the manufacture ofcrosslinked, melt-shaped articles, the process comprising the steps of:

A. Forming a crosslinkable mixture comprising:

-   -   1. Organopolysiloxane containing two or more functional end        groups;    -   2. Polyolefin;    -   3. Silane; and    -   4. Peroxide;

B. Melt-shaping the mixture into an article at conditions sufficient tograft the silane to the polyolefin and to partially crosslink thesilane-grafted polyolefin; and

C. Cooling and continuing the crosslinking of the article.

This embodiment combines the silane grafting of the polyolefin and theinitiation of the crosslinking of the mixture into a single step.

In one embodiment the invention is a process for the manufacture ofcrosslinked, melt-shaped articles, the process comprising the steps of:

-   -   1. Preparing a silane-grafted polyolefin;    -   2. Mixing the silane-grafted polyolefin with a        hydroxy-terminated polydimethylsiloxane;    -   3. Melt-shaping the mixture into a storage article;    -   4. Introducing the storage article to a second melt-shaping        operation in which the storage article is melt-shaped into a        finished article;    -   5. Introducing a crosslinking catalyst during or after the        second melt-shaping operation; and    -   6. Cooling and crosslinking the finished article from the second        melt-shaping operation.        This embodiment allows for the decoupling of the mixture-forming        steps from the melt-shaping and crosslinking steps thus allowing        the process to be performed over different spaces and times. The        storage article is typically pellets which are re-melted and        optionally mixed with a crosslinking catalyst to form the        finished molded or extruded article.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph reporting the data from a dynamic mechanical analysis(DMA) of an ENGAGE plastomer and an ENGAGE plastomer reactively modifiedwith hydroxyl-terminated polydimethylsiloxane (PDMS).

FIG. 2 is a schematic of a cross-section of a molded electricalconnector comprising a thick-wall insulation layer sandwiched betweentwo semiconductive layers.

FIG. 3 is a graph reporting the DMA of the insulation layer of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Unless stated to the contrary, implicit from the context, or customaryin the art, all parts and percents are based on weight and all testmethods are current as of the filing date of this disclosure. Forpurposes of United States patent practice, the contents of anyreferenced patent, patent application or publication are incorporated byreference in their entirety (or its equivalent US version is soincorporated by reference) especially with respect to the disclosure ofsynthetic techniques, definitions (to the extent not inconsistent withany definitions specifically provided in this disclosure), and generalknowledge in the art.

The numerical ranges in this disclosure are approximate, and thus mayinclude values outside of the range unless otherwise indicated.Numerical ranges include all values from and including the lower and theupper values, in increments of one unit, provided that there is aseparation of at least two units between any lower value and any highervalue. As an example, if a compositional, physical or other property,such as, for example, molecular weight, viscosity, melt index, etc., isfrom 100 to 1,000, it is intended that all individual values, such as100, 101, 102, etc., and sub ranges, such as 100 to 144, 155 to 170, 197to 200, etc., are expressly enumerated. For ranges containing valueswhich are less than one or containing fractional numbers greater thanone (e.g., 1.1, 1.5, etc.), one unit is considered to be 0.0001, 0.001,0.01 or 0.1, as appropriate. For ranges containing single digit numbersless than ten (e.g., 1 to 5), one unit is typically considered to be0.1. These are only examples of what is specifically intended, and allpossible combinations of numerical values between the lowest value andthe highest value enumerated, are to be considered to be expresslystated in this disclosure. Numerical ranges are provided within thisdisclosure for, among other things, the component amounts of thecomposition and various process parameters.

“Cable” and like terms mean at least one wire or optical fiber within aprotective insulation, jacket or sheath. Typically, a cable is two ormore wires or optical fibers bound together, typically in a commonprotective insulation, jacket or sheath. The individual wires or fibersinside the jacket may be bare, covered or insulated. Combination cablesmay contain both electrical wires and optical fibers. The cable, etc.can be designed for low, medium and high voltage applications. Typicalcable designs are illustrated in U.S. Pat. Nos. 5,246,783, 6,496,629 and6,714,707.

“Polymer” means a compound prepared by reacting (i.e., polymerizing)monomers, whether of the same or a different type. The generic termpolymer thus embraces the term “homopolymer”, usually employed to referto polymers prepared from only one type of monomer, and the term“interpolymer” as defined below.

“Interpolymer” and “copolymer” mean a polymer prepared by thepolymerization of at least two different types of monomers. Thesegeneric terms include both classical copolymers, i.e., polymers preparedfrom two different types of monomers, and polymers prepared from morethan two different types of monomers, e.g., terpolymers, tetrapolymers,etc.

“Ethylene polymer”, “polyethylene” and like terms mean a polymercontaining units derived from ethylene. Ethylene polymers typicallycomprise at least 50 mole percent (mol %) units derived from ethylene.

“Ethylene-vinylsilane polymer” and like terms mean an ethylene polymercomprising silane functionality. The silane functionality can be theresult of either polymerizing ethylene with a vinyl silane, e.g., avinyl trialkoxy silane comonomer, or, grafting such a comonomer onto anethylene polymer backbone as described, for example, in U.S. Pat. No.3,646,155 or 6,048,935.

“Blend,” “polymer blend” and like terms mean a blend of two or morepolymers. Such a blend may or may not be miscible. Such a blend may ormay not be phase separated. Such a blend may or may not contain one ormore domain configurations, as determined from transmission electronspectroscopy, light scattering, x-ray scattering, and any other methodknown in the art.

“Composition” and like terms mean a mixture or blend of two or morecomponents. For example, in the context of preparing a silane-graftedethylene polymer, a composition would include at least one ethylenepolymer, at least one vinyl silane, and at least one free radicalinitiator. In the context of preparing a cable sheath or other articleof manufacture, a composition would include an ethylene-vinylsilanecopolymer, a catalyst cure system and any desired additives such aslubricants, fillers, anti-oxidants and the like.

“Ambient conditions” and like terms means temperature, pressure andhumidity of the surrounding area or environment of an article. Theambient conditions of a typical office building or laboratory include atemperature of 23° C. and atmospheric pressure.

“Catalytic amount” means an amount of catalyst necessary to promote thecrosslinking of an ethylene-vinylsilane polymer at a detectable level,preferably at a commercially acceptable level.

“Crosslinked”, “cured” and similar terms mean that the polymer, beforeor after it is shaped into an article, was subjected or exposed to atreatment which induced crosslinking and has xylene or decaleneextractables of less than or equal to 90 weight percent (i.e., greaterthan or equal to 10 weight percent gel content).

“Crosslinkable”, “curable” and like terms means that the polymer, beforeor after shaped into an article, is not cured or crosslinked and has notbeen subjected or exposed to treatment that has induced substantialcrosslinking although the polymer comprises additive(s) or functionalitywhich will cause or promote substantial crosslinking upon subjection orexposure to such treatment (e.g., exposure to water).

“Melt-shaped” and like terms refer to an article made from athermoplastic composition that has acquired a configuration as a resultof processing in a mold or through a die while in a melted state. Themelt-shaped article may be at least partially crosslinked to maintainthe integrity of its configuration. Melt-shaped articles include wireand cable sheaths, compression and injection molded parts, sheets,tapes, ribbons and the like.

Ethylene Polymers

The polyethylenes used in the practice of this invention, i.e., thepolyethylenes that contain copolymerized silane functionality or aresubsequently grafted with a silane, can be produced using conventionalpolyethylene polymerization technology, e.g., high-pressure,Ziegler-Natta, metallocene or constrained geometry catalysis. In oneembodiment, the polyethylene is made using a high pressure process. Inanother embodiment, the polyethylene is made using a mono- orbis-cyclopentadienyl, indenyl, or fluorenyl transition metal (preferablyGroup 4) catalysts or constrained geometry catalysts (CGC) incombination with an activator, in a solution, slurry, or gas phasepolymerization process. The catalyst is preferablymono-cyclopentadienyl, mono-indenyl or mono-fluorenyl CGC. The solutionprocess is preferred. U.S. Pat. No. 5,064,802, WO93/19104 and WO95/00526disclose constrained geometry metal complexes and methods for theirpreparation. Variously substituted indenyl containing metal complexesare taught in WO95/14024 and WO98/49212.

In general, polymerization can be accomplished at conditions well-knownin the art for Ziegler-Natta or Kaminsky-Sinn type polymerizationreactions, that is, at temperatures from 0-250° C., preferably 30-200°C., and pressures from atmospheric to 10,000 atmospheres (1013megaPascal (MPa)). Suspension, solution, slurry, gas phase, solid statepowder polymerization or other process conditions may be employed ifdesired. The catalyst can be supported or unsupported, and thecomposition of the support can vary widely. Silica, alumina or a polymer(especially poly(tetrafluoroethylene) or a polyolefin) arerepresentative supports, and desirably a support is employed when thecatalyst is used in a gas phase polymerization process. The support ispreferably employed in an amount sufficient to provide a weight ratio ofcatalyst (based on metal) to support within a range of from 1:100,000 to1:10, more preferably from 1:50,000 to 1:20, and most preferably from1:10,000 to 1:30. In most polymerization reactions, the molar ratio ofcatalyst to polymerizable compounds employed is from 10-12:1 to 10-1:1,more preferably from 10⁻⁹:1 to 10⁻⁵:1.

Inert liquids serve as suitable solvents for polymerization. Examplesinclude straight and branched-chain hydrocarbons such as isobutane,butane, pentane, hexane, heptane, octane, and mixtures thereof; cyclicand alicyclic hydrocarbons such as cyclohexane, cycloheptane,methylcyclohexane, methylcycloheptane, and mixtures thereof;perfluorinated hydrocarbons such as perfluorinated C₄₋₁₀ alkanes; andaromatic and alkyl-substituted aromatic compounds such as benzene,toluene, xylene, and ethylbenzene.

The ethylene polymers useful in the practice of this invention includeethylene/α-olefin interpolymers having a α-olefin content of betweenabout 15, preferably at least about 20 and even more preferably at leastabout 25, wt % based on the weight of the interpolymer. Theseinterpolymers typically have an α-olefin content of less than about 50,preferably less than about 45, more preferably less than about 40 andeven more preferably less than about 35, wt % based on the weight of theinterpolymer. The α-olefin content is measured by ¹³C nuclear magneticresonance (NMR) spectroscopy using the procedure described in Randall(Rev. Macromol. Chem. Phys., C29 (2&3)). Generally, the greater theα-olefin content of the interpolymer, the lower the density and the moreamorphous the interpolymer, and this translates into desirable physicaland chemical properties for the protective insulation layer.

The α-olefin is preferably a C₃₋₂₀ linear, branched or cyclic α-olefin.Examples of C₃₋₂₀ α-olefins include propene, 1-butene,4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene,1-tetradecene, 1-hexadecene, and 1-octadecene. The α-olefins also cancontain a cyclic structure such as cyclohexane or cyclopentane,resulting in an α-olefin such as 3-cyclohexyl-1-propene (allylcyclohexane) and vinyl cyclohexane. Although not α-olefins in theclassical sense of the term, for purposes of this invention certaincyclic olefins, such as norbornene and related olefins, particularly5-ethylidene-2-norbornene, are α-olefins and can be used in place ofsome or all of the α-olefins described above. Similarly, styrene and itsrelated olefins (for example, α-methylstyrene, etc.) are α-olefins forpurposes of this invention. Illustrative ethylene polymers includeethylene/propylene, ethylene/butene, ethylene/1-hexene,ethylene/1-octene, ethylene/styrene, and the like. Illustrativeterpolymers include ethylene/propylene/1-octene,ethylene/propylene/butene, ethylene/butene/1-octene,ethylene/propylene/diene monomer (EPDM) and ethylene/butene/styrene. Thecopolymers can be random or blocky.

The ethylene polymers used in the practice of this invention can be usedalone or in combination with one or more other ethylene polymers, e.g.,a blend of two or more ethylene polymers that differ from one another bymonomer composition and content, catalytic method of preparation, etc.If the ethylene polymer is a blend of two or more ethylene polymers,then the ethylene polymer can be blended by any in-reactor orpost-reactor process. The in-reactor blending processes are preferred tothe post-reactor blending processes, and the processes using multiplereactors connected in series are the preferred in-reactor blendingprocesses. These reactors can be charged with the same catalyst butoperated at different conditions, e.g., different reactantconcentrations, temperatures, pressures, etc, or operated at the sameconditions but charged with different catalysts.

Examples of ethylene polymers made with high pressure processes include(but are not limited to) low density polyethylene (LDPE), ethylenesilane reactor copolymer (such as SiLINK® made by The Dow ChemicalCompany), ethylene vinyl acetate copolymer (EVA), ethylene ethylacrylate copolymer (EEA), and ethylene silane acrylate terpolymers.

Examples of ethylene polymers that can be grafted with silanefunctionality include very low density polyethylene (VLDPE) (e.g.,FLEXOMER® ethylene/1-hexene polyethylene made by The Dow ChemicalCompany), homogeneously branched, linear ethylene/α-olefin copolymers(e.g., TAFMER® by Mitsui Petrochemicals Company Limited and EXACT® byExxon Chemical Company), homogeneously branched, substantially linearethylene/α-olefin polymers (e.g., AFFINITY® and ENGAGE® polyethyleneavailable from The Dow Chemical Company), and ethylene block copolymers(e.g., INFUSE® polyethylene available from The Dow Chemical Company).The more preferred ethylene polymers are the homogeneously branchedlinear and substantially linear ethylene copolymers. The substantiallylinear ethylene copolymers are especially preferred, and are more fullydescribed in U.S. Pat. Nos. 5,272,236, 5,278,272 and 5,986,028.

Silane Functionality

Any silane that will effectively copolymerize with ethylene, or graft toand crosslink an ethylene polymer, can be used in the practice of thisinvention, and those described by the following formula are exemplary:

in which R¹ is a hydrogen atom or methyl group; x and y are 0 or 1 withthe proviso that when x is 1, y is 1; m and n are independently aninteger from 1 to 12 inclusive, preferably 1 to 4, and each R″independently is a hydrolyzable organic group such as an alkoxy grouphaving from 1 to 12 carbon atoms (e.g. methoxy, ethoxy, butoxy), aryloxygroup (e.g. phenoxy), araloxy group (e.g. benzyloxy), aliphatic acyloxygroup having from 1 to 12 carbon atoms (e.g. formyloxy, acetyloxy,propanoyloxy), amino or substituted amino groups (alkylamino,arylamino), or a lower alkyl group having 1 to 6 carbon atoms inclusive,with the proviso that not more than one of the three R groups is analkyl. Such silanes may be copolymerized with ethylene in a reactor,such as a high pressure process. Such silanes may also be grafted to asuitable ethylene polymer by the use of a suitable quantity of organicperoxide, either before or during a shaping or molding operation.Additional ingredients such as heat and light stabilizers, pigments,etc., also may be included in the formulation. The phase of the processduring which the crosslinks are created is commonly referred to as the“cure phase” and the process itself is commonly referred to as “curing”.Also included are silanes that add to unsaturation in the polymer viafree radical processes such as mercaptopropyl trialkoxysilane.

Suitable silanes include unsaturated silanes that comprise anethylenically unsaturated hydrocarbyl group, such as a vinyl, allyl,isopropenyl, butenyl, cyclohexenyl or gamma-(meth)acryloxy allyl group,and a hydrolyzable group, such as, for example, a hydrocarbyloxy,hydrocarbonyloxy, or hydrocarbylamino group. Examples of hydrolyzablegroups include methoxy, ethoxy, formyloxy, acetoxy, proprionyloxy, andalkyl or arylamino groups. Preferred silanes are the unsaturated alkoxysilanes which can be grafted onto the polymer or copolymerizedin-reactor with other monomers (such as ethylene and acrylates). Thesesilanes and their method of preparation are more fully described in U.S.Pat. No. 5,266,627 to Meverden, et al. Vinyl trimethoxy silane (VTMS),vinyl triethoxy silane, vinyl triacetoxy silane, gamma-(meth)acryloxypropyl trimethoxy silane and mixtures of these silanes are the preferredsilane crosslinkers for use in this invention. If filler is present,then preferably the crosslinker includes vinyl trialkoxy silane.

The amount of silane crosslinker used in the practice of this inventioncan vary widely depending upon the nature of the polymer, the silane,the processing or reactor conditions, the grafting or copolymerizationefficiency, the ultimate application, and similar factors, but typicallyat least 0.5, preferably at least 0.7, weight percent is used.Considerations of convenience and economy are two of the principallimitations on the maximum amount of silane crosslinker used in thepractice of this invention, and typically the maximum amount of silanecrosslinker does not exceed 5, preferably it does not exceed 3, weightpercent.

The silane crosslinker is grafted to the polymer by any conventionalmethod, typically in the presence of a free radical initiator, e.g.peroxides and azo compounds, or by ionizing radiation, etc. Organicinitiators are preferred, such as any one of the peroxide initiators,for example, dicumyl peroxide, di-tert-butyl peroxide, t-butylperbenzoate, benzoyl peroxide, cumene hydroperoxide, t-butyl peroctoate,methyl ethyl ketone peroxide, 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane,lauryl peroxide, and tert-butyl peracetate. A suitable azo compound is2,2-azobisisobutyronitrile. The amount of initiator can vary, but it istypically present in an amount of at least 0.04, preferably at least0.06, parts per hundred resin (phr). Typically, the initiator does notexceed 0.15, preferably it does not exceed about 0.10, phr. The weightratio of silane crosslinker to initiator also can vary widely, but thetypical crosslinker:initiator weight ratio is between 10:1 to 500:1,preferably between 18:1 and 250:1. As used in parts per hundred resin orphr, “resin” means the olefinic polymer.

While any conventional method can be used to graft the silanecrosslinker to the polyolefin polymer, one preferred method is blendingthe two with the initiator in the first stage of a reactor extruder,such as a Buss kneader. The grafting conditions can vary, but the melttemperatures are typically between 160 and 260° C., preferably between190 and 230° C., depending upon the residence time and the half life ofthe initiator.

Copolymerization of vinyl trialkoxysilane crosslinkers with ethylene andother monomers may be done in a high-pressure reactor that is used inthe manufacture of ethylene homopolymers and copolymers with vinylacetate and acrylates.

Polyfunctional Organopolysiloxane with Functional End Groups

The oligomers containing functional end groups useful in the presentprocess comprise from 2 to 100,000 or more units of the formula R₂SiO inwhich each R is independently selected from a group consisting of alkylradicals comprising one to 12 carbon atoms, alkenyl radicals comprisingtwo to about 12 carbon atoms, aryls, and fluorine substituted alkylradicals comprising one to about 12 carbon atoms. The radical R can be,for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl,dodecyl, vinyl, allyl, phenyl, naphthyl, tolyl, and3,3,3-trifluoropropyl. Preferred is when each radical R is methyl.

In one embodiment, the organopolysiloxane containing one or morefunctional end groups is a hydroxyl-terminated polydimethylsiloxanecontaining at least two hydroxyl end groups. Such polydimethylsiloxanesare commercially available, for example as silanol-terminatedpolydimethylsiloxane from Gelest, Inc. However, polydimethylsiloxaneshaving other terminal groups that can react with grafted silanes may beused e.g. polydimethylsiloxanes with amine end groups and the like. Inaddition, the polysiloxane may be a moisture-crosslinkable polysiloxane.In preferred embodiments, the polydimethylsiloxane is of the formula

in which Me is methyl and n is in the range of 2 to 100,000 or more,preferably in the range of 10 to 400 and more preferably in the range of20 to 120. Examples of suitable polyfunctional organopolysiloxanes arethe silanol-terminated polydimethylsiloxane DMS-15 (Mn of 2,000-3,500,viscosity of 45-85 centistokes, —OH level of 0.9-1.2%) from GelestCorp., and Silanol Fluid 1-3563 (viscosity 55-90 centistokes, —OH levelof 1-1.7%) from Dow Corning Corp. In some embodiments the polyfunctionalorganopolysiloxane comprises branches such as those imparted byMe-SiO_(3/2) or SiO_(4/2) groups (known as Tor Q groups to those skilledin silicone chemistry).

The amount of polyfunctional organopolysiloxane used in the practice ofthis invention can vary widely depending upon the nature of the polymer,the silane, the polyfunctional organopolysiloxane, the processing orreactor conditions, the ultimate application, and similar factors, buttypically at least 0.5, preferably at least 2, weight percent is used.Considerations of convenience and economy are two of the principallimitations on the maximum amount of polyfunctional organopolysiloxaneused in the practice of this invention, and typically the maximum amountof polyfunctional organopolysiloxane does not exceed 20, preferably itdoes not exceed 10, weight percent.

Crosslinking Catalyst

Crosslinking catalysts include the Lewis and Brønsted acids and bases.Lewis acids are chemical species that can accept an electron pair from aLewis base. Lewis bases are chemical species that can donate an electronpair to a Lewis acid. Lewis acids that can be used in the practice ofthis invention include the tin carboxylates such as dibutyl tindilaurate (DBTDL), dimethyl hydroxy tin oleate, dioctyl tin maleate,di-n-butyl tin maleate, dibutyl tin diacetate, dibutyl tin dioctoate,stannous acetate, stannous octoate, and various other organo-metalcompounds such as lead naphthenate, zinc caprylate and cobaltnaphthenate. DBTDL is a preferred Lewis acid. Lewis bases that can beused in the practice of this invention include, but are not limited to,the primary, secondary and tertiary amines. These catalysts aretypically used in moisture cure applications.

Brønsted acids are chemical species that can lose or donate a hydrogenion (proton) to a Brønsted base. Brønsted bases are chemical speciesthat can gain or accept a hydrogen ion from a Brønsted acid. Brønstedacids that can be used in the practice of this invention includesulfonic acid.

The minimum amount of crosslinking catalyst used in the practice of thisinvention is a catalytic amount. Typically this amount is at least 0.01,preferably at least 0.02 and more preferably at least 0.03, weightpercent (wt %) of the combined weight of ethylene-vinylsilane polymerand catalyst. The only limit on the maximum amount of crosslinkingcatalyst in the ethylene polymer is that imposed by economics andpracticality (e.g., diminishing returns), but typically a generalmaximum comprises less than 5, preferably less than 3 and morepreferably less than 2, wt % of the combined weight of ethylene polymerand condensation catalyst.

Fillers and Additives

The composition from which the crosslinked article, e.g., cableinsulation layer or protective jacket, injection molded elastomericconnector, etc., or other article of manufacture, e.g., seal, gasket,shoe sole, etc., is made can be filled or unfilled. If filled, then theamount of filler present should preferably not exceed an amount thatwould cause unacceptably large degradation of the electrical and/ormechanical properties of the silane-crosslinked, ethylene polymer.Typically, the amount of filler present is between 2 and 80, preferablybetween 5 and 70, weight percent (wt %) based on the weight of thepolymer. Representative fillers include kaolin clay, magnesiumhydroxide, silica, calcium carbonate and carbon blacks. The filler mayor may not have flame retardant properties. In a preferred embodiment ofthis invention in which filler is present, the filler is coated with amaterial that will prevent or retard any tendency that the filler mightotherwise have to interfere with the silane cure reaction. Stearic acidis illustrative of such a filler coating. Filler and catalyst areselected to avoid any undesired interactions and reactions, and thisselection is well within the skill of the ordinary artisan.

The compositions of this invention can also contain additives such as,for example, antioxidants (e.g., hindered phenols such as, for example,IRGANOX™ 1010 a registered trademark of Ciba Specialty Chemicals),phosphites (e.g., IRGAFOS™ 168 a registered trademark of Ciba SpecialtyChemicals), UV stabilizers, cling additives, light stabilizers (such ashindered amines), plasticizers (such as dioctylphthalate or epoxidizedsoy bean oil), scorch inhibitors, mold release agents, tackifiers (suchas hydrocarbon tackifiers), waxes (such as polyethylene waxes),processing aids (such as oils, organic acids such as stearic acid, metalsalts of organic acids), oil extenders (such as paraffin oil and mineraloil), colorants or pigments to the extent that they do not interferewith desired physical or mechanical properties of the compositions ofthe present invention. These additives are used in amounts known tothose versed in the art.

Liquid Polymer Modifier

In an embodiment, the process includes adding a liquid polymer modifierduring the manufacture process of the crosslinked, melt-shaped article.A “liquid polymer modifier,” as used herein, is a non-functionalizedplasticizer (NFP). As used herein, an “NFP” is a hydrocarbon liquid,which does not include to an appreciable extent functional groupsselected from hydroxide, aryls and substituted aryls, halogens, alkoxys,carboxylates, esters, carbon unsaturation, acrylates, oxygen, nitrogen,and carboxyl. By “appreciable extent,” it is meant that these groups andcompounds comprising these groups are not deliberately added to the NFP,and if present at all, are present in embodiments at less than 5 percentby weight of the NFP, or less than 4, 3, 2, 1, 0.7, 0.5, 0.3, 0.1, 0.05,0.01, or 0.001 wt %, based upon the weight of the NFP.

In an embodiment, aromatic moieties (including any compound whosemolecules have the ring structure characteristic of benzene,naphthalene, phenanthrene, anthracene, etc.) are substantially absentfrom the NFP. In another embodiment, naphthenic moieties (including anycompound whose molecules have a saturated ring structure such as wouldbe produced by hydrogenating benzene, naphthalene, phenanthrene,anthracene, etc.) are substantially absent from the NFP. By“substantially absent,” it is meant that these compounds are not addeddeliberately to the compositions and if present at all, are present atless than 0.5 wt %, preferably less than 0.1 wt % by weight of the NFP.

In another embodiment, the NFP does not contain olefinic unsaturation toan appreciable extent. By “appreciable extent of olefinic unsaturation”it is meant that the carbons involved in olefinic bonds account for lessthan 10% of the total number of carbons in the NFP, preferably less than8%, 6%, 4%, 2%, 1%, 0.7%, 0.5%, 0.3%, 0.1%, 0.05%, 0.01%, or 0.001%. Insome embodiments, the percent of carbons of the NFP involved in olefinicbonds is between 0.001 and 10% of the total number of carbon atoms inthe NFP, preferably between 0.01 and 5%, preferably between 0.1 and 2%,more preferably between 0.1 and 1%.

In an embodiment, the liquid polymer modifier is an NFP that is aphthalate-free hydrogenated C₈ to C₁₂ poly-alpha-olefin. Thephthalate-free hydrogenated C₈ to C₁₂ poly-alpha-olefin is naturallyinert and does not affect the cure chemistry of the crosslinkablemixture as do conventional modifiers like mineral oil, white oil andparaffinic oils. Similarly, the present liquid polymer modifier does notaffect other chemistries, such as, for example, antioxidant chemistry,filler chemistry, adhesion chemistry or the like.

In addition, the present liquid polymer modifier has high permanence,good compatibility with polyethylenes and ethylene copolymers, andnarrow molecular weight distribution (Mw/Mn or MWD). As a result,applications using the present liquid polymer modifier have a surprisingcombination of desired properties including high cure efficiency,improved flexibility and toughness and easy processing. Suchapplications display excellent surface properties and exceptionalretention of properties over time.

A nonlimiting example of a suitable liquid polymer modifier is polymermodifier sold under the tradename Elevast, such as Elevast R-150.Elevast polymer modifier is available from the ExxonMobil ChemicalCompany, Houston, Tex.

The liquid polymer modifier advantageously replaces oil extenders(paraffin oil and/or mineral oil) in the crosslinked, melt-shapedarticle. When compared to the same crosslinked, melt-shaped article withoil extender; a crosslinked, melt-shaped article containing the presentliquid polymer modifier unexpectedly exhibits improved softness (i.e.,lower Shore A Hardness value), increased flexibility, (i.e., increase inM100), greater elongation, enhanced elasticity, and improvedprocessability (lower viscosity)—all with no decrease in dielectricstrength of the crosslinked, melt-shaped article. The foregoing physicalimprovements from the liquid polymer modifier are surprising andunexpected in view of conventional oil extenders because oil extendersdecrease dielectric strength in the resultant crosslinked product.Nonlimiting applications of crosslinked, melt-shaped article containingthe present liquid polymer modifier and exhibiting the foregoingphysical improvements (without loss of dielectric strength) include wireand cable, and other applications where good dielectric properties arerequired.

The liquid polymer modifier may be added during different steps of theproduction process. In an embodiment, the liquid polymer modifier isadded to a crosslinkable mixture composed of (1) organopolysiloxane(with two or more hydroxyl end groups) and (2) a silane-grafted orsilane-copolymerized polyolefin. This crosslinkable mixture issubsequently melt-shaped, partially crosslinked, cooled, and furthercross-linked upon exposure to ambient conditions.

In an embodiment, the liquid polymer modifier is added to acrosslinkable mixture composed of (1) organopolysiloxane containing twoor more hydroxyl end groups, (2) polyolefin, (3) silane, and (4)peroxide. The crosslinkable mixture is subsequently melt-shaped,partially crosslinked, cooled and further crosslinked when exposed toambient conditions.

In an embodiment, the liquid polymer modifier is added with thecrosslinking catalyst. A silane-grafted polyolefin is prepared to whicha hydroxyl-terminated polydimethylsiloxane is added. The mixture ismelt-shaped into a storage article. The storage article is introducedinto a second melt-shaping operation wherein the storage article ismelt-shaped into a finished article. The process includes introducingthe crosslinking catalyst and the liquid polymer modifier during orafter the second melt-shaping operation. The process further includescooling and crosslinking the finished article from the secondmelt-shaping operation.

Compounding/Fabrication

Compounding of the silane-functionalized ethylene polymer,polyfunctional organopolysiloxane, catalyst, and filler and additives,if any, can be performed by standard means known to those skilled in theart. Examples of compounding equipment are internal batch mixers, suchas a Banbury or Bolling internal mixer. Alternatively, continuous singleor twin screw mixers can be used, such as a Farrel continuous mixer, aWerner and Pfleiderer twin screw mixer, or a Buss kneading continuousextruder. The type of mixer utilized, and the operating conditions ofthe mixer, will affect properties of the composition such as viscosity,volume resistivity, and extruded surface smoothness.

The components of the composition are typically mixed at a temperatureand for a length of time sufficient to fully homogenize the mixture butinsufficient to cause the material to gel. The catalyst is typicallyadded to ethylene-vinylsilane polymer but it can be added before, withor after the additives, if any. Typically, the components are mixedtogether in a melt-mixing device. The mixture is then shaped into thefinal article. The temperature of compounding and article fabricationshould be above the melting point of the ethylene-vinylsilane polymerbut below about 250° C.

In some embodiments, either or both of the catalyst and the additivesare added as a pre-mixed masterbatch. Such masterbatches are commonlyformed by dispersing the catalyst and/or additives into an inert plasticresin, e.g., a low density polyethylene. Masterbatches are convenientlyformed by melt compounding methods.

In one embodiment, one or more of the components are dried beforecompounding, or a mixture of components is dried after compounding, toreduce or eliminate potential scorch that may be caused from moisturepresent in or associated with the component, e.g., filler. In oneembodiment, crosslinkable silicone-modified polyolefin mixtures areprepared in the absence of a crosslinking catalyst for extended shelflife, and the crosslinking catalyst is added as a final step in thepreparation of a melt-shaped article.

Articles of Manufacture

In one embodiment, the composition of this invention can be applied to acable as a sheath or insulation layer in known amounts and by knownmethods (for example, with the equipment and methods described in U.S.Pat. Nos. 5,246,783 and 4,144,202). Typically, the composition isprepared in a reactor-extruder equipped with a cable-coating die andafter the components of the composition are formulated, the compositionis extruded over the cable as the cable is drawn through the die. Curemay begin in the reactor-extruder.

One of the benefits of this invention is that the shaped article doesnot require post-shaping, e.g., after de-molding or passing through ashaping die, cure conditions, e.g., temperature above ambient and/ormoisture from an external source such as a water bath or “sauna”. Whilenot necessary or preferred, the shaped article can be exposed to eitheror both elevated temperature and external moisture and if an elevatedtemperature, it is typically between ambient and up to but below themelting point of the polymer for a period of time such that the articlereaches a desired degree of crosslinking. The temperature of anypost-shaping cure should be above 0° C.

Other articles of manufacture that can be prepared from the polymercompositions of this invention include fibers, ribbons, sheets, tapes,tubes, pipes, weather-stripping, seals, gaskets, hoses, foams, footwearand bellows. These articles can be manufactured using known equipmentand techniques.

Nonlimiting embodiments of the present disclosure are provided below.

E1. A process for the manufacture of crosslinked, melt-shaped articlesis provided. The process comprises the steps of:

A. Forming a crosslinkable mixture comprising:

-   -   1. Organopolysiloxane containing two or more functional end        groups; and    -   2. Silane-grafted or silane-copolymerized polyolefin;

B. Melt-shaping and partially crosslinking the mixture into an article;and

C. Cooling and continuing crosslinking the melt-shaped article.

E2. The process of E1 in which a crosslinking catalyst is added to themixture before or during melt-shaping or to the melt-shaped article. E3.The process of any of E1-E2 in which at least one of the functional endgroups of the organopolysiloxane is a hydroxyl group. E4. The process ofany of E1-E3 in which the crosslinkable mixture comprises a liquidpolymer modifier. E5. The process of any of E1-E4 in which thepolyolefin is a polyethylene. E6. The process of any of E1-E5 in whichthe catalyst is a Lewis or Brønsted acid or base. E7. The process of anyof E1-E6 in which the crosslinkable mixture comprises, based on theweight of the mixture:

A. 0.5 to 20 wt % of the organopolysiloxane; and

B. 0.01 to 0.2 wt % of the catalyst.

E8. The process of any of E1-E7 in which the crosslinkable mixturefurther comprises at least one of a filler, plasticizing agent, scorchretardant and moisture source. E9. The process of any of E1-E8 in whichat least one of the crosslinkable mixture or a component of the mixtureis subjected to drying conditions prior to melt shaping thecrosslinkable mixture. E10. The process of any of E1-E9 in which atleast one of the organopolysiloxane and catalyst is at least partiallysoaked into the silane-grafted or silane-copolymerized polyolefin at atemperature below the melting temperature of the polyolefin prior tomelt-shaping the mixture.

Another process for the manufacture of crosslinked, melt-shaped articlesis provided (E11) and the process comprises the steps of:

A. Forming a crosslinkable mixture comprising:

-   -   1. Organopolysiloxane containing one or more functional end        groups;    -   2. Polyolefin;    -   3. Silane; and    -   4. Peroxide;

B. Melt-shaping the mixture into an article at conditions sufficient tograft the silane to the polyolefin and to partially crosslink thesilane-grafted polyolefin; and

C. Cooling and continuing the crosslinking of the article.

E12. The process of E11 wherein the crosslinkable mixture comprises aliquid polymer modifier.

Another process for the manufacture of crosslinked, melt-shaped articlesis provided (E13), the process comprising the steps of:

-   -   1. Preparing a silane-grafted polyolefin;    -   2. Mixing the silane-grafted polyolefin with a        hydroxy-terminated polydimethylsiloxane;    -   3. Melt-shaping the mixture into a storage article;    -   4. Introducing the storage article to a second melt-shaping        operation in which the storage article is melt-shaped into a        finished article;    -   5. Introducing a crosslinking catalyst during or after the        second melt-shaping operation; and    -   6. Cooling and crosslinking the finished article from the second        melt-shaping operation.

E14. The process of E13 comprising introducing, with the crosslinkingcatalyst, a liquid polymer modifier.

E15. The process of any of E1-14 in which the mixture is melt-shaped bymolding.

E16. The process of any of E1-14 in which the mixture is melt-shaped byextrusion.

E17. A thick-walled article made by the process of any of E1-14.

E18. An electric power cable comprising an insulation layer made by theprocess of any of E1-14.

E19. An electric power cable accessory or molded connector comprising aninsulation layer made by the process of any of E1-14.

The invention is described more fully through the following examples.Unless otherwise noted, all parts and percentages are by weight.

SPECIFIC EMBODIMENTS Example 1

Table 1 reports the evaluation of several compositions. ENGAGE™ 8200plastomer (an ethylene-octene copolymer of 5 MI, 0.870 density, solidpellets) is used in the experiments. The polymer pellets are heated at40° C. for two hours then tumble blended with a mixture of VTMS andLUPEROX 101 peroxide (2,5-dimethyl-2,5-di(t-butylperoxy)hexane availablefrom Arkema) and left to soak in a glass jar using a jar roller untilthe pellets are visibly dry.

A Brabender batch mixer (250 gram) is used for grafting VTMS to thepolymer. Compounding is conducted at 190° C. for 15 minutes. The graftedpolymer is pressed into a plaque at room temperature and sealed in afoil bag for subsequent experiments with polydimethylsiloxane (PDMS).

A Brabender mixer (45 cc) is used to compound the grafted resin,silanol-terminated PDMS and catalyst. Compounding was performed at a settemperature of 150° C. as follows: First, the mixer was loaded withVTMS-grafted ENGAGE 8200, is fluxed and mixed for 2 minutes at 45revolutions per minute (rpm). Silanol-terminated PDMS (Gelest DMS-S15)is added gradually over a period of approximately 3 minutes and afteraddition is completed, the blend is further mixed for 2 minutes at 45rpm. Catalysts (DBTDL, sulfonic acid or mixture) are then added andmixed for 15 minutes at 45 rpm. If the resulting compound isthermoplastic, i.e. no significant crosslinking is visible, it ispressed into a 50 mil (˜1.3 mm) plaque immediately after removal fromthe mixer and stored overnight in a sealed aluminum foil bag at 25° C.

Samples are then cut to analyze for cure via hot creep analysis (200° C.oven, 15 min). Percent elongation under 20N/mm² load is then measured. Acommon standard for adequate crosslinking is elongation of less than orequal to (≦) 100%. Measurements are obtained on triplicate samples.

TABLE 1 Hot Creep Test Results of Test Compositions Component A B C D EF Si-g-PE 0 99.85 95 94.85 94.85 99.85 Sil-PDMS 5 0 5 5 5 0 Sulfonic 0 00 0 0.15 0.15 Acid. DBTDL 0 0.15 0 0.15 0 0 ENGAGE 95 0 0 0 0 0 8200Total 100 100 100 100 100 100 Total Mixing 22 15 15 21 21 15 Time (min)Hot Creep Melted Fail Fail *Cross- Pass Fail (100% linked Elongation)prematurely *Since the sample crosslinked prematurely, the catalystlevel was subsequently reduced as described in later examples. Si-g-PEis silane grafted ENGAGE 8200 plastomer. Sil-PDMS is Gelest DMS-S15silanol-terminated PDMS. Sulfonic acid is B-201 available from KingIndustries. DBTDL is FASTCAT 4202 dibutyl tin dilaurate. Hot Creep TestPercent Elongation measured at 200° C., 0.2 MPa load held for 15 minutesby IEC 60811-2-1.

As shown by the hot creep test results in Table 1, the addition of PDMSto either the base resin (sample A, a control) or a silane grafted resin(sample B) does not produce the desired cross-linking. Furthercomparative example (sample F), which represent conventional moisturecure, either failed the hot creep test after overnight storage with noexternal moisture exposure (except what may have been trapped duringcompounding or in the storage bag). Inventive samples D and E) in whichOH-terminated PDMS is added to a grafted resin and further reacted witha catalyst produce effective crosslinking, either immediately during thecompounding step in the mixer (sample D) or produced a thermoplasticcompound, that could be shaped into a formed article (e.g. a plaque) andwhen stored overnight in sealed bag produced a homogenous crosslinkingas shown by sample E. This is the desired result.

The data also shows that it is possible to design compositions that canbe homogenously mixed to produce a thermoplastic material that exhibitexcellent crosslinking without the need for external moisture exposurewhich is desirable for thick articles such as molded parts or mediumvoltage and high voltage cable coating.

As a further confirmation of crosslinking, the composition of sample Eis repeated in another experiment, the sample made is subjected to a DMAanalysis, with a temperature sweep from −150° C. to 200° C. As the datain the Figure shows, compared to the ENGAGE 8200 base resin (meltingpoint ˜70° C.), the modulus of the reactively-modified PDMS-ENGAGE blendexhibits a plateau past the melting point, indicating a good temperatureresistance compared to the base resin.

Electron microscopy shows drastically improved phase compatibility. Forexample, sample E shows a predominantly single homogeneous phase withsome finely dispersed silicone domains. In contrast, other compositionstested (samples A and C) show morphologies typical of highly immisciblesystems containing distinct, large domains of silicone visible asdroplets within the polyolefin matrix.

Example 2

The data reported in Table 2 compares an LLDPE resin (0.7 MI, 0.920g/cm³ density) grafted with 2% VTMS in the presence of 3%silanol-terminated polydimethylsiloxane (OH-PDMS) versus a controlsample grafted under the same conditions without the OH-PDMS. Bothmaterials are first dried and then extruded on a wire (124 mil wireO.D., 30 mil wall thickness) in the presence of a tin catalyst. Theinsulation is removed, cured for 16 hours under ambient conditions (23 Cand 70% relative humidity), and then subjected to a hot creep test at200° C., 15 min, 15 N/m²). The results show that the comparativecomposition does not achieve 100% hot creep elongation and 10% hot settargets. In contrast, the inventive composition does pass the hot creepand hot set tests. The data demonstrate the rapid cure rate at ambientconditions achieved with the invention.

TABLE 2 Hot Creep and Hot Set Test Results of Test CompositionsInventive Comparative Composition Composition Hot Creep (% elongation)Pass Fail Hot Set (% elongation) Pass Fail

Example 3

The data set for this example is obtained on a sample taken from amolded part. Molded part 10 (FIG. 2) comprises insulation layer 11 madeout of an elastomer resin system which is grafted withvinyltrimethoxysilane in the presence of OH-PDMS. Molded part 10 is a 35KV prototype connector comprising outer (12) and an inner (13) semiconlayers sandwiching insulation layer 11. Insulation layer 11 comprises acomposition of this invention. The semicon layers are first moldedseparately and peroxide-cured in a first molding step, then mountedtogether in a second mold where the insulation layer is injected betweenthem. The insulation compound is premixed with a tin catalystmasterbatch, injection is conducted in a fully thermoplastic fashion,and the part is de-molded upon cooling (1-5 minutes molding timedepending on the test run). Inner semicon layer 13 is about 4 mm thickand covers most of the insulation, except towards the ends. Outersemicon layer 12 is about 3.5 mm thick and covers all the insulationlayer, i.e. no external exposure, and insulation layer 11 itself isabout 11.6 mm thick. Once received from the molding shop, the part iscut and three samples are taken from the middle section of theinsulation layer for DMA testing. All samples are 1.9 mm thick. Startingfrom the outside edge of the insulation layer, Sample 1 is about 3 mminside the layer, Sample 2 is about 5 mm inside the layer, and Sample 3is about 7 mm inside the layer. The part is handled under normalshipping and lab storage conditions prior to testing, i.e. no specialheat or moisture exposure. The DMA data in FIG. 3 shows a plateaumodulus at a temperature above the melting point for each of the samplesor in other words, complete cure of the material.

Although the invention has been described with certain detail throughthe preceding specific embodiments, this detail is for the primarypurpose of illustration. Many variations and modifications can be madeby one skilled in the art without departing from the spirit and scope ofthe invention as described in the following claims.

What is claimed is:
 1. A process for the manufacture of crosslinked,melt-shaped articles, the process comprising the steps of: A. Forming acrosslinkable mixture comprising:
 1. at most 10 wt %, based on the totalweight of the crosslinkable mixture, of an organopolysiloxane containingtwo functional end groups which are hydroxyl groups, wherein theorganopolysiloxane is a polydimethylsiloxane of the formula

wherein Me is methyl and n is from 10 to 400; and
 2. Silane-graftedpolyethylene; B. Melt-shaping and partially crosslinking the mixtureinto an article; and C. Cooling the melt-shaped article; and D. Storingthe melt-shaped article and continuing crosslinking without externalmoisture diffusion, wherein a crosslinking catalyst is added to themixture before or during melt-shaping or to the melt-shaped article,wherein the melt-shaped article has a thickness of greater than 0.2 mm,and wherein the crosslinked, melt-shaped article passes the hot creeptest (100% elongation) measured at 200° C., 0.2 MPa load held for 15minutes in accordance with IEC 60811-2-1.
 2. The process of claim 1 inwhich the catalyst is a Lewis or Bronsted acid or base.
 3. The processof claim 1 in which the crosslinkable mixture comprises, based on theweight of the mixture: i. 0.5 to 10 wt % of the organopolysiloxane; andii. 0.01 to 0.2 wt % of the crosslinking catalyst.
 4. The process ofclaim 1 in which at least one of the crosslinkable mixture or acomponent of the mixture is subjected to drying conditions prior to meltshaping the crosslinkable mixture.
 5. The process of claim 1 in which atleast one of the organopolysiloxane and crosslinking catalyst is atleast partially soaked into the silane-grafted polyethylene at atemperature below the melting temperature of the silane-graftedpolyethylene prior to melt-shaping the mixture.
 6. The process of claim1 in which the crosslinking catalyst is a Bronsted acid.
 7. The processof claim 6 in which the crosslinking catalyst is sulfonic acid.
 8. Theprocess of claim 1 in which the melt-shaped article is a cable coating.